If you don't live in the US of A and would prefer your Firefox browser to be a little more local, you can add local dictionaries such as the British English Dictionary 1.19.1 by Mark Tyndall or the British English Dictionary R1.19 for Firefox, Thunderbird and SeaMonkey by David Bartlett. Both are very handy plugins, very easy to use.
If you go to Tools > Add-Ons > Get Add-Ons, you will find a search box in the top left corner to assist you. Entering the word "dictionary" and a country will likely bring up something for your locale, in order of relevance.
Click the Install button beside the one you want and Firefox will handle the task. You can then switch to the Extensions tab to manage your extensions (enable, disable, remove and configure options if there are any) for the Add-ons you have installed.
Don't ask me why we have two terms extension and add-on for the same objects, maybe someday Mozilla will house-clean that oddity.
Dictionaries generally only work against user-input feilds on web-pages. To enable spell-checking in a particular language, right-click in a multi-line text box, select "Languages", then choose one of your localised languages, such as "English / United Kingdom".
For those wanting a locale dictionary, but Firefox keeps defaulting back to US English, choosing a locale (such as UK English) when installing Firefox would have helped, but after-the-fact, all you need to do is go into Tools > Options >Content for the tab Languages > Choose Language in order of preference: English/United Kingdom [en-gb]. You could remove the default US English if you wish, otherwise making UK English is the default, click OK.
You may find that some of the local dictionaries are less than comprehensive, but then if you've put up with Microsoft's attempts at localisation over the years, you'll have adjusted your expectations. Most will cover the fundamentals of local spellings, even if certain proper names are not recognised - these are all working on fairly small local lexicons and they're free, so what's to complain about? RC